There are good indications that the deep level around 0.45 eV induced by oxygen acts as an effective electrical recombination center in heterojunction bipolar transistors with a graded band‐gap AlGaAs emitter‐base junction. The transistor, with a concentration of the 0.45 eV level on the order of about 5×1015cm−3in the junction region, shows a very small current gain of 3–4, down from more than 30 at a collector current density of 1×103A/cm3. This reduction is greater than the previously reported one by the level approximately 0.6 eV in heterojunction bipolar transistors. The level about 0.45 eV is detected from the junction region that has an oxygen atom concentration of more than 1018cm−3. This level may also act as an optically nonradiative recombination center.